A very low-field (VLF) MRI system employing the signal from hyperpolarized (3)He and (129)Xe will be developed in order to investigate the human lung, and to explore the potential for a low-cost, portable diagnostic tool for the detection and staging of pulmonary diseases. The spin-1/2 nuclei of (3)He and (129)Xe can be "hyperpolarized" to 105 times their thermal equilibrium value spin-exchange collisions with Rb atoms that have been "optically pumped" by circularly polarized light entirely into one electron spin state. The vastly enhanced signal from such hyperpolarized species is strong enough to permit high speed imaging of the space they occupy, making them intriguing MR probes. The field has developed considerably in the past four years, since the PI and collaborators invented hyperpolarized noble gas MRI by acquiring MR images of an excised mouse lung that had been inflated with hyperpolarized (129)Xe. Improved lung gas-space imaging has been demonstrated using the signal from both hyperpolarized (3)He and (129)Xe. With the latter, there is the promise of investigating both ventilation and perfusion since it is highly soluble in blood and tissues. Of special importance is the high SNR achievable in hyperpolarized noble gas MRI, at VLF. VLF MRI becomes feasible since the level of polarization of the nuclei is set by the hyperpolarization process, and is independent of the static magnetic field, B(o). The signal strength therefore scales as B(o) rather than as the B(o)(2) of conventional MRI. The SNR then becomes virtually independent of the field over a great range, and low field strength is not a disadvantage. Very low-fields of about 100-200 Gauss (0.01-0.02 T) can be produced by lightweight solenoids at low-cost. Therefore, VLF noble gas MRI may lead to inexpensive, portable imaging systems. The development of a VLF MRI system is proposed using custom-made resistive magnet of 100-200 Gauss. Gradient, shim, and RF coils will be developed and interfaced to a commercial low frequency MRI console. A spatial resolution on the order of 0.5 mm is planned over a field of view of 30 cm DSV, the typical size of human lungs. Since sample noise is dominated by coil noise at VLF, high temperature superconducting (HTS) coils will be developed to improve SNR and in-plane resolution. Pulse sequences will be designed for very low-field acquisition and optimized in order to take advantage of the large, but non-renewable hyperpolarized magnetization that is characteristic of hyperpolarized noble gas MRI. Preliminary human lung imaging studies will be conducted. A long term objective is the development of a low-cost portable unit that can be used for lung screening in a variety of clinics, physician's offices and nursing homes, and perhaps even for space based pulmonary function research in microgravity environments.